In the previous video we learned that unit tests demonstrate that the code 0:00 written up to a given point is correct and they give us confidence to move on. 0:04 There are many more reasons to unit test. 0:10 Unit tests help to discover design flaws early in the coding process. 0:11 Unit test should always be written by the developer of the code. 0:17 Often, when we start coding we may not be thinking about 0:21 all the possible ways that the code could fail. 0:24 There are often corner cases that we may not have considered. 0:27 This can affect how the software is designed and coded. 0:31 When writing unit tests, we're forced to think about these things and we end up 0:35 with a more intimate knowledge of the code and the problem we're trying to solve. 0:39 Unit tests find runtime errors. 0:44 Runtime errors can cause the software to crash or to stop working. 0:47 In C# the most common example of a runtime error is an improperly handled exception. 0:51 By testing the code at the unit level, we can get much better visibility regarding 0:57 where exceptions may arise and how they should be handled. 1:03 In scripted and non-compiled languages such as Python or JavaScript, unit 1:06 tests are used to discover coding errors that could have been caught by a compiler. 1:11 Unit tests find logic errors. 1:16 Just because the software appears to work on the surface and 1:18 doesn't crash, doesn't mean that it's correct. 1:21 The software may still produce the wrong output or 1:24 display the wrong information to the user. 1:28 This is most likely due to errors in the way that 1:30 the logic of the software was coded. 1:33 When doing unit tests, we try to exercise the code 1:36 as many different ways as possible by providing it different sets of inputs. 1:39 For each input, we check to see if we get the expected output. 1:44 Unit tests provide multiple entry points into the code. 1:48 The entry point of a piece of software is where the code starts executing 1:52 when it's launched. 1:56 For example, the entry point of a console application is always the main method. 1:57 Unit tests give us the ability to run every unit of code 2:02 independently of the rest of the software. 2:05 This in essence provides multiple entry points into the code 2:08 that we wouldn't otherwise have. 2:11 This makes it very easy to isolate and debug the code, when we discover problems. 2:14 This is especially important when coding a library, 2:19 because libraries don't have entry points. 2:22 Unit tests are part of the documentation of the code. 2:25 When working with code, that you didn't write yourself. 2:28 It's important to have some sort of documentation, to learn how to use it. 2:30 If you're lucky, someone will have taken the time to write some good documentation. 2:34 Documentation can be hard to keep up to date though. 2:39 In the absence of good documentation, you can find examples of how to use the code 2:42 by looking at the unit test, provided that the developer wrote unit tests of course. 2:47 Running unit tests is easy to automate because they are written in code and 2:51 saved in files. 2:56 They can be run and re-run as often as we want. 2:57 We'll see many examples of that in this course. 3:00 The best developers in the world regularly make mistakes and that's okay. 3:03 Even the most experienced developers would never overlook testing though. 3:08 There's an old saying that claims an ounce of prevention is worth a pound of cure, 3:12 and that is so true for unit testing. 3:17 It often shortens the overall time it takes to finish a product 3:20 by building quality in early in the process. 3:23 At the very least, 3:26 it reduces the amount of uncertainty in a project, especially at the end. 3:27 With all of these benefits, it's easy to see why unit testing is a critical 3:31 part of the software development process. 3:35